In a wire bonding process, electrically conductive wires are bonded between electrical contact pads found on a semiconductor die and leads on a substrate onto which the die is attached, usually a semiconductor leadframe. The wire needs to be held firmly, fed to the bonding site and stripped off at appropriate junctures in the process. This is usually achieved using a wire clamp. Over the years, the operational speed of wire bonding machines has increased considerably, with the result that the wire clamp needs to be actuated at high speed while exerting controlled force on the wire being clamped, without damaging the wire.
Modern day wire bonders for making so-called “ball-bonds” are designed to execute a rocking motion of a bond-body which carries a bonding tool, about a suitably located pivot. Since the wire clamp is generally carried on the rocking bond-body, it needs to be made as light as possible. Its inertia about the bond-body pivot axis needs to be as small as possible in order to enable high speed bonding operation without need for an unduly large actuator or motor for actuating movement of the bond-body. Even so, the wire clamp needs to have high static and dynamic stiffness thus giving rise to high resonant frequencies of vibration. This ensures that any residual vibration of the wire clamp at the end of the bond-body stroke is of low amplitude and high frequency, and that it settles fast enough, to enable high speed bonding without adversely affecting the bond quality.
In the past, a variety of actuation methods such as voice coil motors, solenoids, piezo-electric actuators, magnetostrictive actuators and others, have been used to actuate wire clamps in wire bonders.
U.S. Pat. Nos. 3,672,556 and 4,142,714 disclose similar variations of a solenoid actuated wire clamp. These designs are of a “normally open” type meaning that if the power to the solenoid is cut off, the clamp remains in the open position, thereby unclamping the wire. Present-day wire bonders demand a “normally closed” type wire clamp. Also, the designs in the aforesaid patents require numerous parts to transmit the actuation force from the solenoid to the clamping location. This makes it cumbersome for present-day high speed wire clamping wherein the clamp may need to operate at a rate of about 20 times per second or even more. At this speed of operation, the long term reliability of the clamp is also questionable, since it contains several parts which slide against each other, thus leading to friction and wear.
Several designs of piezo-electric wire clamps have also been patented, such as, for example, in U.S. Pat. Nos. 5,901,896, 5,388,751 and 5,314,175. These involve expensive piezo-electric actuator elements and compliant structures made using expensive wire EDM (Electro Discharge Machining). The operating voltages for piezo-electric actuators, in the range of 100-200 volts, are much higher than those for electromagnetic actuators (eg. solenoids and voice coil motors).
FIG. 1 shows a commonly used voice coil motor actuated wire clamp. Wire 1 is clamped between damper plates 2 and 3 affixed to the ends of a movable arm 4 and fixed arm 5 respectively. The wire clamp is mounted on a bond-body through mount holes 6 on the fixed arm 5. A voice coil motor 7 is used to actuate the movable arm 4 with respect to the fixed arm 5. The movable arm 4 is pivotally mounted on the fixed arm 5 using smooth and hard pivot ball bearings 8 made of wear-free material, eg. ruby. Extension spring 9 provides a small initial bias force (also called “preload force”) between the movable and fixed arms 4, 5. The extension spring 9 is located on the same side of the pivot balls 8, as the damper plates 1, 2, thus ensuring that the clamp is normally closed. When the coil of the voice coil motor 7 is energized by an electric current in one direction, a force is exerted on the movable arm 4 such that the movable arm 4 rotates about pivot balls 8 in the direction indicated by arrow F, thus opening the wire clamp. On de-energizing the coil, the spring force of spring 9, rotates the movable arm 4 about the pivot balls 8 in the direction opposite to arrow F, thus closing the wire clamp. When the coil is energized by an electric current in the opposite direction, the motor force tends to increase the clamping force on the wire 1 between damper plates 2 and 3. The clamp opening stroke of the movable arm 4, is limited by including a hard stopper ball 10 to come into contact with a hardened surface 11 of the fixed arm 5 at a fully-opened position.
As seen in the above description, such a wire clamp has numerous components and uses a spring loaded pivot ball bearing 8. The ball bearing 8 on its own does not contribute to a force to close the wire clamp, thus requiring a spring 9 in addition to the force generated by the voice coil motor 7 to contribute to clamping force. The use of a voice coil motor 7 involves a bulkier device and makes operation of the wire clamp relatively more complex.